Extension method to reveal TextMeshPro characters over time. Requires UniTask.

RevealTextWithDelay.cs

using System.Collections.Generic;
using System.Threading;

using Cysharp.Threading.Tasks;
using TMPro;

public static class TextMeshProExtensions
{
    private static Dictionary<TextMeshProUGUI, CancellationTokenSource> textToCancel = new Dictionary<TextMeshProUGUI, CancellationTokenSource>();

    /// <summary>
    /// Reveals characters of the given text with a delay between each character.
    /// </summary>
    /// <param name="milliseconds">Milliseconds between each character being shown.</param>
    /// <param name="padding">Adds this to the number of characters to be revealed. Can be used if the text might
    /// change while the reveal is happening to make sure all the characters are available.</param>
    /// <example>
    /// yourTMProText.RevealTextWithDelay(20).Forget();
    /// </example>
    public static async UniTaskVoid RevealTextWithDelay(this TextMeshProUGUI proText, int milliseconds = 10, int padding = 0)
    {
        // If this text is already in the middle of revealing text, cancel its currently running
        // task so that the reveal process can be restarted.
        if (textToCancel.ContainsKey(proText))
        {
            textToCancel[proText].Cancel();
            textToCancel.Remove(proText);
        }

        // Create a cancellation token that can be called to interrupt this text in case it's told
        // to reveal text while in the middle of already revealing it. Combo it with the typical
        // Unity OnDestroy token to handle the usual case of the object disappearing while running.
        var interruptShowingTextCancel = new CancellationTokenSource();
        var superCancel = CancellationTokenSource.CreateLinkedTokenSource(
            interruptShowingTextCancel.Token,
            proText.GetCancellationTokenOnDestroy());
        textToCancel.Add(proText, superCancel);

        var lastUpdateTime = System.DateTime.Now;
        int visibleCount = 0;
        while (visibleCount < proText.text.Length + padding)
        {
            // Since frame times can be longer than the number of milliseconds desired between
            // characters, account for how many letters would have passed since last cycle.
            var timeSinceLast = System.DateTime.Now.Subtract(lastUpdateTime);
            int extraCharacters = timeSinceLast.Milliseconds / milliseconds;

            visibleCount += 1 + extraCharacters;
            proText.maxVisibleCharacters = visibleCount;
            lastUpdateTime = System.DateTime.Now;

            await UniTask.Delay(milliseconds, cancellationToken: superCancel.Token);
        }
    }
}